This invention relates generally to internal combustion engines that propel motor vehicles and are equipped both with a variable valve actuation mechanism and with an exhaust system having a particulate filter for trapping particulates in engine exhaust. A more particular aspect of the invention relates to a control strategy for regenerating the particulate filter through use of the variable valve actuation mechanism.
A particulate, or soot, filter may be placed in the exhaust system of an engine, especially a diesel engine, to trap soot that would otherwise be emitted to the surrounding atmosphere. As the engine accumulates more operating time, more soot becomes trapped in the filter. Eventually the amount of accumulated soot begins to affect performance, and so it becomes necessary to purge the filter of trapped soot.
Because the soot is essentially carbon, a known process for purging the soot is to combust it into carbon dioxide by temporarily elevating the temperature of the exhaust passing though it to a sufficiently high temperature and providing sufficient oxygen. This process is known as regeneration. A known technique for elevating the temperature of the exhaust is by altering the timing and the amount of fueling of the engine.
It is believed that the present invention provides a better strategy for regenerating the filter. When an engine has a variable valve actuation mechanism under control of a processor-based engine control, the timing of intake valve opening and closing and the timing of exhaust valve opening and closing can be varied. The invention arises from the discovery that the variable valve actuation mechanism can be used to vary the exhaust temperature. In particular, it has been found that reducing the amount of valve overlap during a certain portion of the engine operating cycle can temporarily elevate engine exhaust temperature sufficiently to regenerate a soot filter that has been trapping soot in the exhaust. As a piston is approaching top dead center (TDC) to complete an exhaust stroke, the exhaust valve for the corresponding cylinder is beginning to close. It attains full closure at or near TDC. At or near TDC, the corresponding intake valve is beginning to open. Valve overlap occurs during that fraction of the engine cycle when both valves are simultaneously open. By decreasing the amount of valve overlap, i.e. by decreasing the fraction of an engine operating cycle for which both intake and exhaust valves for a respective cylinder are simultaneously open as a piston in the respective cylinder approaches top dead center to complete an exhaust stroke, the temperature of the flow leaving the cylinders and passing through the exhaust system can be elevated to a temperature that is effective to combust particles that have been trapped by the filter, thereby regenerating the filter.
Supplemental heating of the exhaust may be appropriate if the exhaust temperature is below a low temperature limit when valve overlap is initially decreased. The supplemental heating represents an additional action that can be taken by altering the injection of fuel in conjunction with the decrease in valve overlap.
Principles of the invention can be embodied in an engine control as part of an overall engine control strategy. The creation of an engine and an operating strategy that can regenerate a particulate filter in conjunction with an overall control strategy, is seen to be a desirable objective because it can make the regeneration process transparent to an operator of the vehicle, requiring no special measures on his or her part. The present invention attains the desired objective through the control of the variable actuation mechanism for the engine cylinder valves by an engine control system embodying solid-state, processor-based electronics. The processor can process certain data inputs to develop data outputs defining valve opening and closing times that are most appropriate to the inventive strategy. The data outputs are converted into electric signals that are delivered via electric circuits to actuators of the variable valve actuation mechanism to open and close the intake and exhaust valves at proper times for accomplishing regeneration.
Because certain principles of the present invention include changing the time in the engine operating cycle when the intake and exhaust valves for the engine cylinders open and close, the variable valve actuation mechanism of the engine must be effective on each such cylinder valve. An example of such a mechanism comprises an electric actuator for opening and closing each corresponding cylinder valve in accordance with the electric signal applied to the actuator. This type of engine is sometimes referred to as a camless engine.
The invention is believed to provide an advantageous solution for soot filter regeneration because it is possible to implement in an existing engine at no added hardware cost by providing the processor with an operating program embodying an appropriate algorithm, provided that the engine, has a processor-based control and a variable valve actuation mechanism. To the extent that the disclosed embodiment uses certain data inputs for the operating program, they are already available. The invention is also believed to be more fuel-efficient than the known method referred to above.
Accordingly, a fundamental aspect of the present invention relates to a novel system and strategy for regeneration of a soot filter in an exhaust system of an internal combustion engine, especially a compression ignition, or diesel, engine that also has a variable valve actuation mechanism for varying the timing of cylinder intake and exhaust valves. The invention comprises causing the variable valve actuation mechanism to change the timing of the engine cylinder valves during the engine cycle in a manner that causes elevation of the temperature of flow through the exhaust system to a temperature that is effective to combust particles that have been trapped by the filter, thereby regenerating the filter.
One general aspect of the claimed invention relates to a method of regenerating a particulate filter that traps combustible particulates in an exhaust system of an internal combustion engine that has a variable valve actuation mechanism for varying the opening and closing times of exhaust valves that control flow from cylinders of the engine into the exhaust system and for varying the opening and closing times of intake valves that control flow into the cylinders from an intake system of the engine. A first more particular aspect of the method comprises, with the engine running under its own power, operating the variable valve actuation mechanism to decrease the fraction of an engine operating cycle for which both intake and exhaust valves for a respective cylinder are simultaneously open as a piston in the respective cylinder approaches top dead center to complete an exhaust stroke, and as a result, elevate the temperature of flow through the exhaust system to a temperature that is effective to combust particles that have been trapped by the filter, thereby regenerating the filter. A second more particular aspect of the method comprises, with the engine running under its own power, operating the variable valve actuation mechanism to advance the closing of the intake valves toward bottom dead center (BDC), and as a result, elevate the temperature of flow through the exhaust system to a temperature that is effective to combust particles that have been trapped by the filter, thereby regenerating the filter. These two more particular aspects may be utilized independently or conjunctively.
Still another aspect relates to an engine that embodies one or both of these more particular aspects.
Another general aspect relates to a control for controlling a variable valve actuation mechanism of an internal combustion engine to regenerate a particulate filter in the exhaust system of such an engine. The control comprises an operating program for regenerating the particulate filter while the engine is running under its own power by causing the variable valve actuation mechanism to decrease the fraction of an engine operating cycle for which both intake and exhaust valves for a respective cylinder are simultaneously open as a piston in the respective cylinder approaches top dead center to complete an exhaust stroke, and as a result, elevate the temperature of flow through the exhaust system to a temperature that is effective to combust particles that have been trapped by the filter, thereby regenerating the filter.
The foregoing, along with further aspects, features, and advantages of the invention, will be seen in this disclosure of a presently preferred embodiment of the invention depicting the best mode contemplated at this time for carrying out the invention. This specification includes drawings, briefly described below, and contains a detailed description that will make reference to those drawings.